It is a known bug present since ages ago. The bug is that when someone else activates another beacon, the creatures stop spawning. And for people that visit Nimbus every day, its important, because that is one of the "big" missions on there. But since Cryptic will never do anything to fix it, people just stop writting about it since time ago. The Orion contraband mission is also bugged.
Picard: "The economics of the future is somewhat different. You see, money doesn't exist in the 24th century... The acquisition of wealth is no longer the driving force in our lives. We work to better ourselves and the rest of Humanity."
/ Tell that to the Ferengi..